1. Field of the Invention
The present invention relates to a transfer member employed for transferring toner in an image forming apparatus such as a laser printer utilizing electrophotography, for example.
2. Description of Related Art
In an image forming apparatus such as a laser printer, an electrostatic copier, a plain paper facsimile or a composite machine thereof utilizing electrophotography, an image is formed on the surface of a paper (including a plastic film such as an OHP film: this also applies to the following description) generally through the following steps:
First, a photosensitive body having photoconductivity is prepared, and the surface of the photosensitive body is exposed in a uniformly charged state. Thus, an electrostatic latent image corresponding to the image to be formed is formed on the surface of the photosensitive body (charging step→exposing step).
Then, toner consisting of fine coloring particles is brought into contact with the surface of the photosensitive body in a state previously charged to a prescribed potential. Thus, the toner is selectively adhered to the surface of the photosensitive body in response to the potential pattern of the electrostatic latent image, which in turn is developed into a toner image (developing step).
Then, the toner image is transferred to the surface of the paper (transfer step) and further fixed (fixing step), thereby forming the image on the surface of the paper.
In the transfer step, the toner image formed on the surface of the photosensitive body may not only be directly transferred to the surface of the paper, but may also be temporarily transferred to the surface of an image carrier (primary transfer step) and thereafter retransferred to the surface of the paper (secondary transfer step).
In order to transfer the toner image from the surface of the photosensitive body to the surface of the paper in the transfer step, from the surface of the photosensitive body to the surface of the image carrier in the primary transfer step or from the surface of the image carrier to the surface of the paper in the secondary transfer step, a transfer member such as a transfer roller including a cylindrical roller body made of a semiconductive rubber composition is widely employed.
In order to transfer the toner image from the surface of the photosensitive body to the surface of the paper with the transfer roller in the transfer step, for example, the paper is passed through the space between the photosensitive body and the transfer roller brought into pressure contact with each other with prescribed force, while a prescribed transfer voltage is applied therebetween. Thus, the toner image formed on the surface of the photosensitive body is transferred to the surface of the paper.
A transfer roller including a roller body formed by blending an electronically conductive filler such as conductive carbon black or an ion-conductive polymer such as epichlorohydrin rubber into crosslinkable rubber and kneading the materials for preparing a rubber composition, cylindrically extrusion-molding the rubber composition and thereafter crosslinking the same, for example, is widely employed as the transfer roller.
A transfer roller including a roller body brought into a porous structure by introducing a foaming agent into the rubber composition and foaming the foaming agent before or simultaneously with crosslinking of the rubber composition is also widely employed.
The roller body may have a single-layer structure consisting of the rubber composition, or a multilayer structure obtained by stacking another layer on the outer periphery or the inner periphery of a layer made of the rubber composition.
For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 9-114189 (1997)) describes a technique of employing a roller including a roller body having a multilayer structure obtained by stacking an outer layer made of a rubber composition made of mixed rubber of acrylonitrile-butadiene rubber (NBR) and SBR and containing conductive carbon black or perchlorate on the outer periphery of an inner layer made of a rubber composition prepared by blending conductive carbon black into NBR, ethylene-propylene-diene rubber (EPDM) or silicone rubber as a transfer roller.
Patent Document 2 (Japanese Unexamined Patent Publication No. 2002-278320) describes a technique of employing a roller including a roller body having a multilayer structure obtained by stacking an outer layer made of a fluoric material or the like on the outer periphery of an inner layer made of a rubber composition prepared by blending conductive carbon black into a mixture of EPDM, NBR and SBR as a transfer roller for a secondary transfer step.
Patent Document 3 (Japanese Unexamined Patent Publication No. 2009-198768) describes a transfer roller including a roller body made of a rubber composition prepared by blending at least one material selected from a group consisting of NBR, SBR and butadiene rubber (BR) and epichlorohydrin rubber.